The present invention relates to the fabrication of micromechanical structures, and more particularly to fabrication of such structures by techniques derived from semiconductor fabrication processes, or techniques applicable to the production of hybrid semiconductor/mechanical devices such as sensors.
In the last decade or so a number of techniques have been developed for forming mechanical structures of extremely small size by depositing a film, and masking, etching or otherwise patterning such film to produce a diaphragm, force responsive beam or wall, resonant structure, diffusion tunnel or other miniature structure or mechanical element. When produced on semiconductor grade crystalline wafers, these structures can not only enjoy the mechanical strength of single crystal structures, but may include signal conditioning or sensing circuitry integrally fabricated on the wafer. The submicrometer-level pattern forming techniques developed for semiconductor fabrication allow the construction of many such structures or devices on a single wafer, which may then be scored and divided into individual units, and packaged.
As representative of such sensing structures, one may consider oscillating plate type structures, such as shown in U.S. Pat. No. 4,699,006, or more static mechanical elements such as shown in U.S. Pat. No. 4,744,863. The last-mentioned patent shows a diaphragm formed over a closed chamber, with electrical sensing elements fabricated on the diaphragm. Such diaphragms may be of small overall area so that despite their thinness they can withstand high pressures. While such a structure provides an effective pressure sensor mechanism, it would be desirable to provide two-sided protection to protect the diaphragm against pressure spikes when used in a high pressure environment, in a pressure spikes when used in a high pressure environment, in a manner similar to the pressure stops commonly provided in larger mechanical construction of push-pull differential pressure sensors. In general, however, microlithographic techniques are suitable only for forming relatively thin elements on the wafer surface.
To provide a mechanically strong overpressure stop above the diaphragm requires fabricating a metal structure that is considerably thicker than the micrometer-scale thicknesses characteristic of circuit or diaphragm type structures, and the resist-masking techniques that have proven effective for formation of microelectronic elements are inadequate for the much thicker architecture of forming frames, posts, stops and housings, or for fabricating small machine elements such as gears having a thickness of over ten or twenty microns. Thus, at these greater thicknesses considered necessary to provide sufficient stiffness for pressure stops, gears or the like, there is a need for development of thick microstructure fabrication techniques.